This invention relates to apparatus for molding parts from viscous materials and, more particularly, to molding apparatus in which molding compound, or elastomeric stock, is transferred from an injection cylinder to a plurality of closed mold cavities through the intermediary of a plurality of passageways which intercommunicate the cylinder with the cavities.
One example of such a molding apparatus is the invention of Herbert F. Jurgeleit disclosed in U.S. Pat. No. 2,883,704, dated Apr. 28, 1959. In this patent a multi-plate, "flashless" mold is provided having a plurality of cavities therein. The term "flashless mold" as used herein has reference to a multi-cavity, multi-part mold in which the cavity forming portions of the mold are independently movable sufficiently to permit independent stacking or closing of such portions either by reason of an actual flexure of the flexible plate member which either forms or directly supports such portions of the mold, or by reason of a movement of such portions relative to a retainer plate, and in which the force for independently closing or holding such cavity forming portions closed is transmitted to each group of cavity forming portions by a common conformable medium such as the molding compound or other conformable medium. Each cavity of the flashless mold is closed except for the presence of a plurality of passageways or sprues provided in the top plate of the mold which intercommunicate each cavity with the space above the top mold plate. In addition, an injection unit is provided which comprises an injection piston movable in an open-ended cylinder commonly referred to as a "bottomless cylinder". The injection unit is usually affixed to the lower surface of the heated, upper platen of a press and the mold is usually removably placed atop the upper surface of the heated, lower platen or ram cap of the press, directly in line with the injection unit.
In the operation of this apparatus, a quantity of molding compound in excess of that which would be sufficient to fill all the cavities is placed on the top plate of the mold, and the press is closed. During closure, the top plate of the mold contacts the lower end of the bottomless cylinder, closing the lower end thereof. As closure of the press continues, the mold and cylinder move upward relative to the injection piston, causing the molding compound atop the mold to become compressed between the injection piston and the top mold plate and to flow through the various sprues into the respective mold cavities. The excess compound in the injection cylinder is maintained under compression both during filling of the cavities and during the time that it takes for the heated upper and lower platens to cure the compound in the filled cavities. This maintains hydraulic pressure on the top plate of the flashless mold in order to keep the cavities closed during the filling and curing period (limiting flash formation at the mold parting lines) and in order to prevent backrinding or backflow of the compound from the cavities through the sprues (with the creation of attendant, undesirable blemishes on, and/or changes in, the dimensions of the parts being molded in the cavities). In this process, however, the excess compound in the injection cylinder is cured along with the compound in the mold cavities and represents waste which is generally scrapped.
Various attempts have been made to improve on the above prior art procedure in order to overcome the losses inherent in the scrapping of the molding compound which is cured in the injection cylinder. One such attempt culminated in the invention of Herbert F. Jurgeleit disclosed in U.S. Pat. No. 3,070,843, dated Jan. 1, 1963. In that process, a piston-cylinder injection unit is closed by a flexible, metallic plate, which plate not only allows the hydraulic pressure of the molding compound to be transmitted to the top mold plate of a mold (single or multiple cavity, flashless or non-flashless) during the injection cycle (to thereby assist in keeping the mold cavities closed during such injection), but retains the molding compound in the injection unit when the molds are thereafter moved away from the injection unit into a separate heated, curing area. With this arrangement, curing of the excess compound in the injection unit may be avoided. However, this is at the expense of providing a plurality of molds and a separate curing area. Moreover, when the injection cylinder and mold are separated to facilitate movement of the mold to the separate curing area, the pressure in the cavities and on the top mold plate is released, allowing, to some degree, backrinding of compound through the sprues and possible flash formation at the parting lines of the mold.
Another attempt to overcome the losses inherent in the scrapping of molding compound which is cured in the injection cylinder is exemplified by the invention of Herbert F. Jurgeleit disclosed in U.S. Pat. No. 3,121,918, dated Feb. 25, 1964. In this process, a piston-cylinder injection unit, which cooperates with a multi-cavity, flashless mold, is also closed by a flexible, metallic plate but, instead of moving the mold to a separate, heated, curing area to prevent curing of the excess compound in the injection cylinder, the injection cylinder itself is moved to a cool area while the mold remains in the heated area and the compound in the cavities is cured. In this instance, again, the injection pressure is relieved after the cavities have been filled in order to allow removal of the injection cylinder. Accordingly, backrinding may occur at such time, resulting in the formation of blemished parts, parts whose dimensions may fail to match precisely the dimensions of the cavities in which they are formed, and/or parts whose density may be less than that required by the specifications for the parts.